Posted
by
Hemos
on Saturday July 31, 2004 @08:35AM
from the blue-moon-you-saw-me-standing-alone dept.

ArbiterOne writes "Tonight a rare spectacle can be seen: the second full moon in a month, which is sometimes called the "blue moon", according to CNN. Don't be disappointed if it isn't actually blue, though; the blueness is caused by increased density of smoke or ash in the air, such as after a volcanic eruption."

I don't know where this google calculator gets its numbers from, but blue moons are actually twice as common as it implies.

There are approximately 13 lunar cycles in a year, and 12 months. Simple math should be able to tell you that there must be a blue moon on average at least once a year--rarely two, and rarely none. In fact, over the next 20 years, 17 will have blue moons [infoplease.com].

The moon's cycle is about 29.5 days, which makes about 12.4 full moons a year. That means a blue moon about every 3 years.

The 17 blue moons in 20 years is due to two different definitions of what constitutes a blue moon. So, blue moons are twice as common, because there are two different ways to define what a blue moon is!

The lunar cycle is 29.5 days long on average, not 28. That gives about 12.4 full moons per year, on average.

I don't know how infoplease.com counted blue moons to get 17 in the next twenty years, unless it was counting both the 2nd-in-a-month and the 4th-in-a-season varieties. Ask this blue moon calculator [obliquity.com] to list the blue moons between 2004 and 2024, and it lists nine of them (of the 2nd-in-a-month kind).

According to the CNN article the original definition is the 3rd full moon in a quarter that has 4 full moons, so tonight meets both the original Maine Farmer's Almanac definition and the post-1980 Sky and Telescope magazine definition.

The phrase "Once in a blue moon" was first noted in 1824 and refers to occurrences that are uncommon, perhaps even rare. Yet, to have two full Moons in the same month is not as uncommon as one might think. In fact, it occurs, on average, about every 32 months. And in the year 1999 it actually occurred twice in three months.

I heard that the adoption of the term blue moon in modern usage actually stems from a misappropriation of the term from some Native American tribe (around the time frame you mention, early 19th C). The term, as far as my uninformed and caffeine-addled mind can conjure at the moment, originally referred to an occurrence of a particular full moon that occurred during the three-month harvest season; specifically, when four moons occurred during this three month season, the third of the sequence was called a "blue moon". (The Wikipedia entry [wikipedia.org] seems to make reference to this--see the second entry--but I can't find anything specific on it. Also, it doesn't seem to mention the "harvest" season, just any season, which is not what I heard from my unreliable source.)

This is why our concept of blue moon as the second in a month doesn't seem to make sense in terms of rarity...it's not that rare. It makes a bit more sense if we consider how often four moons occur during the three month harvest season. That was probably quite a bit more rare.

Here [com.com] is a really good explanation of the background on our current definition of a "Blue Moon" which seems to have been popularized by none other than Trivial Persuit.

On July 31 of this year, those taking an evening stroll beneath a cloudless sky will have the opportunity to look up and see what modern folklore refers to as a "blue moon." Though the moon itself will not actually be blue--or likely even appear blue--it will be the second full moon in the same calendar month of July, which is what qualifies this Saturday's lunar view as a blue moon, at least by today's standards.

That's not to say that the moon has never appeared blue in color to the average, unaided, ground-bound observer. The moon can and has shone blue or even green in color whenever a sufficient quantity of micron-sized particulates is present in the atmosphere, usually after a volcanic eruption.

The micron-sized debris (specifically, any particles slightly larger than the wavelength of red light, about 0.7 microns) refracts the moonlight, turning it green or blue, based on certain atmospheric factors. For an example of this phenomenon, one need look no further than accounts of blue moons following the eruption of the island of Krakatau in 1883.

While instances of actual blue-tinted moons represent one of several documented meanings of the term, using blue moon to describe the second full moon in one calendar month is a fairly recent practice. This definition arose in the 20th century, and it didn't really become part of the widespread public consciousness until the 1980s.

We can attribute this sudden proliferation of a previously undocumented and unused definition for blue moon to three otherwise reputable sources of fine information and fun trivia, all of which were working off bad information.

WHAT THREE MASS-MEDIA TRIVIA SOURCES ARE MOST RESPONSIBLE FOR THE NEWEST DEFINITION OF A "BLUE MOON"?

What three reputable mass-media trivia resources are most responsible for the current popular definition of a "blue moon" to mean the second full moon in the same calendar month, and what is the likely origin of this "mistaken" definition?

The most recent and most widely known source of the "new" blue moon definition is none other than the king daddy of all trivia board games, Trivial Pursuit. Specifically, the Trivial Pursuit Genus II edition published in 1986 included this definition as a question in the Science and Nature category.

Given the popularity of the game, it's little wonder that folklorists witnessed this definition enter widespread usage in the 1980s. But where did the game makers get their information?

Trivial Pursuit archives (yes, they exist) cite The Kids' World Almanac of Records and Facts, published in 1985, as the source of the question. The book's authors, however, can't trace their own source for this "fact."

So where did the Almanac authors get it? Folklorist Philip Hiscock suggests it came from our second mass-media source.

In January 1980, the National Public Radio (NPR) program "Star Date" featured a piece by Deborah Byrd that noted the "second full moon in one calendar month" definition of blue moon. Byrd cited a 1946 article in Sky & Telescope magazine as her source. Hiscock considers the "Star Date" broadcast as the likely source of the Almanac entry, and thus NPR is the second mass-media source to get blue moon "wrong."

The third, obviously, is the usually reliable Sky & Telescope magazine. A March 1946 article by amateur astronomer James Hugh Pruett titled "Once in a Blue Moon" contains the modern definition of the term, but it cites the 1937 Maine Farmers' Almanac as its source. No edition of that Almanac, however, contains the modern definition of a blue moon.

Several editions of the Almanac do list a different definition of a blue moon--the third, extra full moon of an agricultural season. The Maine Farmers' Almanacs of tha

This is an interesting article. There is more to blue moons than I realized.

It was particularly interesting to me since I just saw Tom Skilling (a Chicago television meteorologist) describe how two blue moons in a month was the wrong definition and four in a season is correct. He even mentioned the 1946 Sky and Telescope article as the reason for the confusion.

I suspect he read this later Sky and Telescope article, but reading it I realize that even his "correction" is bit of an over-simplification.

And of course, in very old trade calendars (1930's or thereabouts IIRC) phases of the moon were prominently displayed with the calendar dates. If a page spilled over to another month, and a full moon was shown, it was invariably printed in blue ink.

The reason I always thought is each of 12 moons has a name like "Harvest Moon," "Hunter's Moon" and the like. But when a month occurs that contains 2 full moons, there is no name for this second moon and it was given the name, "Blue Moon." for lack of a name.

This is almost right, but the way the names of the moons were assigned was more complicated than this. The names were assigned based on the seasons, with three named full moons per season. If a season had four full moons, the extra one had no assigned name and was called a "blue" moon. This is not necessarily the second full moon in a month, however, nor was the "blue" moon necessarily the last one in the season. They had some scheme for figuring out which of the moons got which name. The modern connection

But calling two full moons in one month a 'blue moon' first happened in about the 1920s when the Farmer's Almanac gave each full moon of the year a special name. The event of a double full moon was called a blue moon. Later (around 1985) Trival pursuit made it this into a question and this usage became popular.

widespread adoption of the second-full-Moon-in-a-month definition followed its use on the popular radio program StarDate [stardate.org] on January 31, 1980. We examined this show's script, authored by Deborah Byrd, and found that it contains a footnote not read on the air that cites Pruett's 1946 article as the source for the information. Byrd now writes for the radio program Earth & Sky [earthsky.com], whose Web site contains a few short notes giving her perspective on this modern contribution to lunar folklore.

Rede Me and Be Not Wroth (1528): "Yf they say the mone is blewe/We must believe that it is true." According to the Oxford English Dictionary, "if the moon is blue" is equivalent to saying "if the moon were made of green cheese." In other words, it's meant to indicate a patent absurdity.

So, why would ash make the moon look blue? Wouldn't particles in the air tend to scatter shorter wavelength light more than longer wavelength light, giving the moon a red tinge? I know I've seen plenty of red moons.

We see a full moon every 28 days. So a month with 31 days will have a blue moon if it has a full moon in the first three days. The chance of that is 3/28. The chance of a blue moon in a 30-day month is 2/28. There are 7 31-day months, 4 30-day months. So chance of no blue moon in a year (treating the months as independent of each other which obviously isn't the case but shouldn't affect the outcome) is (25/28)^7 * (26/28)^4 which is about 43.4%. So there's a 56.6% chance of a blue moon in any year.

(treating the months as independent of each other which obviously isn't the case but shouldn't affect the outcome)

It is not the case, it should affect the outcome, and it does. Using your figures and ignoring fractional days of a lunation, the day of the month on which a full moon occurs regresses 1-3 days each month, so if a "blue" moon occurs this month (in days 29-31) one can't occur next month (in days 26-28).

The logic behind your math is incorrect... The months are not independent at all. In fact, they tend to nestle right up against each other.

There's no need to use probability here; enumeration will do quite nicely. It makes things a little easier if we approximate the lunar period as exactly 28 days and pretend that the year starts on March 1st (so that leap years are easier to deal with).

Starting at March 1st, there are 27 days on which the first full moon of the year can fall. It works out as:
M

Using a value of 29.53, there's a 1.47/31 chance in January, March, May, July, September, and December. There's a 0.47/30 chance in June, August, and November, and no chance in February. There's a (0.47-1/24)/(30-1/24) chance in April and a (1.47+1/24)/(31+1/24) chance in October (corrected for DST). This comes out to a probability of about.395 per year.

No, this means that all windows patches released today will actually not cause any more additional vulnerabilities. Too bad this had to happen on a Saturday. Oh well, only 2.7 more years.
- This is my first post

I wonder if the blue effect of ash and smoke would cancel out the yellowish hue of the moon when it's behind thin clouds to produce a somewhat greyscale moon, even when viewed in not-so-perfect weather. (As in color temperature)

According to Canadian folklorist Philip Hiscock, the term "blue Moon" has been around for more than 400 years, but its modern calendrical meaning has become widespread only in the last 25. And as discovered five years ago, it can be traced to a mistake published in Sky & Telescope in the 1940s!

"Black Moon is even more rare: When no full moons occur in a month (usually February)"

That would make it 2018 then?

Blue moon: 31/Jan/2008, 9amWorm moon: 01/Mar/2008, 10pm

b.t.w. I don't think it's "usually february", I think it is always february. Every month other than february is guaranteed to be longer than the synodic period (about 29.5 days), so is guaranteed to contain a full moon. (apart from the month in which you change from julian to gregorian calendars...)

A smiling crescent moon can only be seen in tropical or near tropical latitudes. In order for there to be a vertical crescent moon the sun must be directly below the moon, and this only occurs where the ecleptic is close to vertical. The only time I have seen one is when I was in Hawaii a few years ago.

According to an article on the Cincinnati Enquirer today (http://www.enquirer.com/editions/2004/07/31/loc_b luemoon31.html)

"Only once in a blue moon is there controversy over exactly what constitutes a blue moon. But it's not a blue moon in the strict definition of the 19th Century Farmer's Almanac. According to the almanac, a true blue moon is the third full moon in a season that has four full moons. This also happens about every three years, but on a different cycle from the blue moon that occurs tonigh

That reminds me, in a roundabout way, of one of my favorite jokes. A college psychology professor decides to quiz his students on their sexual habits. He asks the class how many have had sex more than once in the past week? Several raise their hands. How about once in the past week? More hands go up. Once in the past two weeks? And so on... after he has gotten to once every two months, he notices that everyone in the classroom has raised his or her hand, except for one guy sitting in the back. So he goes on:

"Three months". Nothing.

"Four months". Nope.

"Six months". Still nothing.

"One year?!".

Finally the guy in the back practically leaps out of his chair, raising his hand up high and bouncing up and down enthusiatically. The prof acknowledges him, and asks, "Well if you only get it once a year, why are you so happy?". The geeky guy smiles and says:

See the Sky & Telescope article: "What's a Blue Moon? The trendy definition of "blue Moon" as the second full Moon in a month is a mistake."
http://skyandtelescope.com/observing/objects/moon/ article_127_1.asp

This falls under science? This is solely an artifact of our time measuring system. The moon is full every 28 days, but months are slightly longer than that. If a full moon is early enough in the month, it will be full twice.

The number 28 appears to be a common confusion between the length of time between the same phase of the moon and the length of time it takes the moon to revolve once around the earth. . According to this [rhbnc.ac.uk] site:

The period of revolution is the SIDEREAL MONTH and has mean value of 27.32166 days. The SYNODICAL MONTH is the interval of time between two identical phases and has a mean value of 29.53059 days.

A time measurement system based on the frequency vernal equinox, solar days and an attempted reconcilliation of the moon's orbit around the earth with the tropical year. Tidal forces between the earth, moon and sun keep everything related and fairly close to integer numbers.

"The moon is full every 28 days, but months are slightly longer than that."

I **DID** read the article, before anyone jumps on my ass. And I read the part about the first full moon of the month occurring so close to perigee and causing a noticeable variation in the tides. But I wonder if there's a relationship between having this "extra" full moon and tidal variations.

Factors that affect tides are the moon's distance from the earth, and its alignment relative to the sun. Tides are higher when the moon is at perigee (when its orbit brings it closest to the earth). Tides are also higher when the sun, moon, and earth align with the sun and moon on the same side of earth. The gravitational forces due to sun and moon add up to produce higher tides. The latter affect would occur at new moon; don't know if the former has any relationship to the lunar phases. Neither affect woul

This is just a calendar coincidence not any kind of astronomical event. And in fact it happens quite often. So really, there is nothing special to see here. Was that really worth mentioning on slashdot?

We've just got home from a night out, and we noticed that there is a big ring around the moon (at least from here in Melbourne) - very strange. If I was sober I would google this interesting phenomenon - can someone just tell me what it is instead?

... which is bluish due to reflected light from the earth, according to the caption, NOT due to scattering from particles etc.... scattering tends to make light from the Sun, moon etc. RED not blue...
Oh for a physicist when you really need one (line now!).

This whole `second full moon in a month' thing is just stupid. Looks like there's already some interesting comments here about the original source of this idea.

Fortunately, there is a chance of a real blue moon [reuters.com] if you live near Anchorage, Alaska. Actually, it's not really a fortunate thing, because if this volcano does erupt and spew ash everywhere, it's a big mess. It mucks up your car, you don't want to breath it, it's almost caused airplanes to crash, etc. But it does have the possibility of causing the moon to appear truely blue.

Most of the mistakes, commentary, etc. have been captured by previous posters (see notes on APOD picture, Sky and Telescope explaining their mistake that led to the popularization of the mistaken "two full moons in a month" idea, etc. But one problem with the original article remains to be fixed:
Atmospheric scattering tends to make light from the Sun, moon etc. red not blue, if due to macroscopic particles, or just to wash out the sky color if due to water vapor, etc. The sky is blue under good conditio

Those of us on the other side of the dateline won't get our blue moon till the end of August. The full moon didn't officially happen until our August 1st (your July 31st), but luckily it all washes out in the end.

This is hilarious. The idea that a month has to do with moons goes back to calenders that were moon based (Muslim Calender, and Jewish calender is partially). On them, there is exactly one full moon per month, being "full" on the fifteenth (or fourteenth, on the now one-day-off Jewish calender).

The Gregorian and Julian calenders are solar based, however, being they have 31 days in some months, it is possible to have to full moon on both the first and 31st day of the month. That this is a rare occurence, is